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Climate: Soot a factor in declining spring snow cover

Decline of reflective snow cover likely to speed overall warming

Soot is contributing to a steady decline in spring snow cover.

By Summit Voice

SUMMIT COUNTY — Heat-trapping greenhouse gases aren’t the only reason the spring snow cover across the northern hemisphere has been declining steeply the past few decades.

By tweaking a sophisticated set of climate models, researchers found that black carbon and dust — both generated by human activities — are at least part of the reason that spring snow cover in Eurasia is declining faster than across North America.

Declining spring snow cover has a feedback effect of intensifying warming because snow-covered ground reflects incoming radiation. Once the snow is melted, the heat is absorbed.

Some aerosols reflect incoming solar energy, potentially cooling underlying surfaces, but black carbon tends to warm surfaces by absorbing incoming solar energy. Particulates that fall to the surface also reduce snow’s reflective qualities, causing even more radiation to be absorbed.

Generated by human activity, dust storms, and forest fires, Asia produces high levels of both types of aerosols, which blow across the Eurasian land mass and affect the surface and nearby atmosphere in a variety of ways.

In the Northern Hemisphere, spring-time snow cover is unique because of its widespread distribution, and because intense incoming solar radiation during that season amplifies atmospheric aerosols’ effects.

Because higher concentrations of organic matter and black carbon are typical in the atmosphere and on the snow-covered surfaces in Eurasia, Flanner and his colleagues hypothesize that those aerosols might account for regional snow-cover differences. By including black carbon and organic matter aerosols in climate models, the researchers hypothesized that the models might more effectively match spring-time observations.

To test their hypothesis, the team first ran a number of modeling scenarios to see if the inconsistency might relate to ocean-based effects. If oceans proved to have a leading role, the aerosol hypothesis would likely be incorrect. However, after constraining the oceans’ effects, the models continued under-predicting land-surface temperature trends. The findings indicated that a land effect had to account for the discrepancy between observations and models showing warming and melting trends.

Having eliminated ocean effects, the researchers enhanced the models with snow-darkening characteristics, mimicking the impact of dark materials deposited on top of pristine snow. With this adjustment, the models correctly indicated increased springtime warming in Eurasia.

Next, the researchers incorporated human-produced carbon dioxide, or CO2, into the models.  The scientists found that over North America, CO2 had more of an impact on springtime snow cover than black carbon and organic materials, but in Eurasia, as hypothesized, the particulates were far more influential, having as much of an effect as CO2.

“While this research does not fully explain why springtime land temperatures and snow cover are changing so much faster over Eurasia than North America, it does suggest that snow darkening from black carbon, a process lacking in most climate models, is playing a role,” Flanner said.

Ultimately, Flanner said, the magnitude of Earth’s climate response to CO2 and other human-generated products depends on feedbacks. Changes in snow cover amplify initial climate changes and constitute one of the most powerful feedbacks. Because snow covers much of the Northern Hemisphere during spring, Flanner and his colleagues expect to see some of the strongest climate change signals in northerly regions during local spring.

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5 Responses

  1. Black carbon seems to be making the headlines a lot of late. Reducing it would lead to big benefits to us all. Reducing carbon dioxide emissions is still crucial though.
    I can understand the Eurasian snow being exposed to more black carbon than the West Side of North America, but how does the East Coast of the USA compare to Eurasia and the West Coast?

    • … Including health benefits, John. Good questions on the geography. I’ve submitted some email questions to the researchers to get more info and will do a followup story.

      Meanwhile, our snow in the West of the US is not as pristine as one might think. We’ve had serious problems with SW desert dust storms the last few years, also leading to a rapid spring snowmelt.

  2. I will not get involved in the causes of global warming etc. as I am not qualified to answer the many questions which it brings to the table. I think we need to find the right balance in terms of planting many more trees than we destroy. As fantastic carbon controllers, the need for reforestation must be immense on our planet. The good news from the UK this year is that we are doing just that on our tiny little island. We are planting at least one million trees to commemorate our Queen. Mindyou, the time has come for mankind to give nature much more back than it takes.

    Kind Regards

    Tony Powell

  3. Records, facts, data, computer modeling, seem to line up, though there are those who still resist. Interesting thoughts here, as is the reforesting, which surely would seem to be better than stripping the earth of trees, especially in the manner that seems to be preferred, i.e.; clear cutting.

    • Jared Diamond, in “Collapse” which described the disappearance of several civilizations such as Easter Island, Sumeria, etc., drew the conclusion that deforestation was a common element. The loss of trees on our planet from insect infestations, rain forest burning, wildfires associated with draught, harvesting for firewood (Haiti and sub-Saharan Africa) portend a bleak future. We in the Colorado mountains face a dry summer this year and high wildfire risk.

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